1. Field of the Invention
The present invention relates electrically conductive coatings. More particularly, the invention relates to transparent electrically conductive coatings comprising carbon nanotubes.
2. Description of the Related Art
Electrically conductive transparent films are known in the art. In general, such films are generally formed on an electrical insulating substrate by either a dry or a wet process. In the dry process, PVD (including sputtering, ion plating and vacuum deposition) or CVD is used to form a conductive transparent film of a metal oxide type, e.g., tin-indium mixed oxide (ITO), antimony-tin mixed oxide (ATO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (FZO). In the wet process, a conductive coating composition is formed using an electrically conductive powder, e.g., one of the above-described mixed oxides and a binder. The dry process produces a film having both good transparency and good conductivity. However, it requires a complicated apparatus having a vacuum system and has poor productivity. Another problem of the dry process is that it is difficult to apply to a continuous or big substrate such as photographic films or show windows.
The wet process requires a relatively simple apparatus, has high productivity, and is easy to apply to a continuous or big substrate. The electrically conductive powder used in the wet process is a very fine powder having an average primary particle diameter of 0.5 μm or less so as not to interfere with transparency of the resulting film. To obtain a transparent coating film, the conductive powder has an average primary particle diameter of half or less (0.2 μm) of the shortest wave of visible light so as not to absorb visible light, and to controlling scattering of the visible light.
The development of intrinsically conductive organic polymers and plastics has been ongoing since the late 1970's. These efforts have yielded conductive materials based on polymers such as polyanaline, polythiophene, polypyrrole, and polyacetylene. (See “Electrical Conductivity in Conjugated Polymers.” Conductive Polymers and Plastics in Industrial Applications”, Arthur E. Epstein; “Conductive Polymers.” Ease of Processing Spearheads Commercial Success. Report from Technical Insights. Frost & Sullivan; and “From Conductive Polymers to Organic Metals.” Chemical Innovation, Bernhard Wessling.
A significant discovery was that of carbon nanotubes, which are essentially single graphite layers wrapped into tubes, either single walled nanotubes (SWNTs) or double walled (DWNTs) or multi walled (MWNTs) wrapped in several concentric layers. (B. I. Yakobson and R. E. Smalley, “Fullerene Nanotubes: C1,000,000 and Beyond”, American Scientist v.85, July-August 1997). Although only first widely reported in 1991, (Phillip Ball, “Through the Nanotube”, New Scientist, Jul. 6, 1996, p. 28–31.) carbon nanotubes are now readily synthesized in gram quantities in the laboratories all over the world, and are also being offered commercially. The tubes have good intrinsic electrical conductivity and have been used in conductive materials.
U.S. Pat. No. 5,853,877, the disclosure of which is incorporated by reference in its entirety, relates to the use of chemically-modified multiwalled nanotubes (MWNT). The coating and films disclosed in U.S. Pat. No. 5,853,877 are optically transparent when formed as a very thin layer. As the thickness of the films increases to greater than about 5 μm, the films lose their optical properties.
U.S. Pat. No. 5,853,877 also relates to films that are formed with and without binders. The films include binders with a very high nanotube concentration and are extremely thin in order to maintain the optical properties. For example, the patent discloses a film with 40% wt MWNT loading to get good ESD conductivities.
U.S. Pat. No. 5,908,585, the disclosure of which is incorporated by reference in its entirety, relates the use of two conductive additives, both MWNT and an electrically conductive metal oxide powder.